Backlight device

ABSTRACT

A backlight device wherein LEDs which are arranged side by side at a first arrangement interval (P 1 ) are provided to each LED unit. The LED units are mounted to a flexible board at a second arrangement interval. A light guide plate is disposed in such a manner that an end surface thereof is spaced from the LEDs by a distance (H). The light guide plate is disposed so as to satisfy the following relationship: first arrangement interval (P 1 )≦distance (H)≦second arrangement interval (P 2 ). Specifically, the LEDs are arranged side by side in such a manner that the first arrangement interval (P 1 ) is not less than 0.1 mm but not greater than 3 mm and the second arrangement interval (P 2 ) is not less than 5 mm but not greater than 15 mm. The light guide plate is disposed in such a manner that the distance (H) is not less than 1 mm but not greater than 7 mm.

TECHNICAL FIELD

The present invention relates to a backlight device and, moreparticularly, to a backlight device where light is irradiated from therear surface of the image display panel.

BACKGROUND

The development of a flat display utilizing a liquid crystal (LCD)panel, PDP (Plasma Display Panel) and the like are being pursuedactively today. In recent years, each manufacturer particularly directsthe attention to the further thinning of such a flat display. Forexample, it is known that a backlight device irradiating light from therear of a panel is required when a display utilizing the LCD panel is tobe manufactured. To promote the thinning of the display, the thinning ofthe backlight device is also one important subject.

To achieve the thinning of the backlight device, proposed is a backlightsystem where light is emitted from an LED (Light Emitting Diode) towardan end face of a light guide plate (see Patent Document 1, forinstance). Where the LED is placed beside the light guide plate asdescribed above, it is possible to suppress the increase in thickness ofthe backlight device due to the light source. Also, when a separationdistance between an incidence plane of the light guide plate and alight-emitting element is denoted by d and the distance between eachlight-emitting element is denoted by p, a backlight device meeting0.2p≦d≦0.8p is proposed (see Patent Document 2, for instance).

RELATED ART DOCUMENTS Patent Documents

[Patent Document 1] Japanese Unexamined Patent Application Publication(Translation of PCT application) No. 2003-532270.

[Patent Document 2] Japanese Unexamined Patent Application PublicationNo. 2007-234412.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, since LED is a point light source, a plurality of LEDs need tobe arranged along the end face of the light guide plate. In this case,the non-uniformity of luminous intensity caused by the placementinterval of the LEDs must be suppressed. Further, making the size of thedisplay larger is being pursued together with the thinning. Thisrequires an increase in the amount of light emitted from the entirebacklight. As described above, when light is to be emitted toward theend face of the light guide plate, many of the LEDs must be arrangedaround the end face of the light guide plate. However, for example, whentwo or more LEDs are to be grouped and then assembled to facilitate theassembly, LEDs placed side by side around one end face of the lightguide plate may all not be equally spaced. In such a case, thenon-uniformity of luminous intensity caused by placement intervals needsto be suppressed as well.

The present invention has been made to solve problems as describedabove, and a purpose thereof is to suppress the non-uniformity ofluminous intensity caused by the placement interval of LEDs in abacklight device having a plurality of LEDs emitting light to an endface of a light guide plate.

Means for Solving the Problem

To resolve the above-described problems, a backlight device according toone embodiment of the present invention includes: a light guide plate;and a plurality of white light-emitting diodes, placed side by side in adirection extending along an end face of the light guide plate, theplurality of white light-emitting diodes being spaced apart from the endface of the light guide plate by a distance H. The plurality of whitelight-emitting diodes are constituted of a group of two or more whitelight-emitting diodes, and are placed side by side such that a placementinterval between each white light-emitting diode in the group is P1, anda placement interval between each group is P2, and the light guide plateis so arranged as to meet a condition that P1≦H≦P2.

Advantageous Effects

The present invention suppresses the luminance non-uniformity caused bythe placement interval of LEDs in the backlight device having aplurality of LEDs emitting light on the end face of the light guideplate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an image display apparatus accordingto a first embodiment.

FIG. 2 is a diagram showing a backlight device according to a firstembodiment as viewed from a viewpoint P of FIG. 1.

FIG. 3 is a graph showing a relation between a distance H and anon-uniformity of luminous intensity K on an end face of a light guideplate in a backlight device according to a first embodiment.

FIG. 4 is an illustration schematically showing a front view of abacklight device according to a first embodiment.

FIG. 5 is a graph showing a relation between a light guide plate heightL4 and a distance H in a backlight device according to a firstembodiment.

FIG. 6 is a block diagram schematically showing a configuration of alighting control system provided in an image display apparatus accordingto a first embodiment.

FIG. 7 is an illustration schematically showing a front view of abacklight device according to a second embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail based onpreferred embodiments with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a cross-sectional view of an image display apparatus 10according to a first embodiment. FIG. 1 is a cross-section of an upperportion of the image display apparatus 10 cut along a plane vertical tothe horizontal direction of the image display apparatus 10. The imagedisplay apparatus 10 includes a liquid crystal panel 12, a front cover13, a chassis 14, a cushion 16, an optical sheet 18, and a backlightdevice 20.

The crystal display panel 12 is formed in a rectangular plate-likeshape. A light shielding section 12 a is provided on a rim of the liquidcrystal panel 12. The chassis 14 has a cross-section which is so shapedthat the letter L is rotated by 180 degrees, and has a length in thehorizontal direction which is slightly longer than the liquid crystalpanel 12. The chassis 14 is so arranged as to cover a front of deviceand an upside of device above the liquid crystal panel 12. The cushion16 is provided under the front of the chassis 14, and the lightshielding section 12 a located on an upper side of the liquid crystalpanel 12 is interposed between the cushion 16 and the front cover 13.Also, the rear surface of the chassis 14 is subjected to the surfacetreatment in white color to have a higher light reflectance.

The backlight device 20 is installed in a rearward side of the liquidcrystal panel 12 and the chassis 14. The backlight device 20 includes alight guide plate 22, a rear plate 24, a heatsink 26, a heat radiationsheet 28, a base plate unit 30, and a reflection sheet 40. Similarly, aheatsink 26, a heat radiation sheet 28, and a base plate unit 30 arealso provided in a lower side of the backlight device 20. Hereinafter,the components only in the upper portion of the backlight device 20 aredescribed, and the description of the heatsink 26, the heat radiationsheet 28 and the base plate unit 30 provided in the lower side thereofis omitted. The placement positions of the heatsink 26, the heatradiation sheet 28 and the base plate unit 30 are not limited to theupside of device and the downside of device of the light guide plate 22.For example, the heatsink 26, the heat radiation sheet 28 and the baseplate unit 30 may be placed on the right side and the left side of thelight guide plate 22 as viewed from the front of the image displayapparatus 10.

The light guide plate 22 is formed of a plate-like member and sizedslightly larger than the liquid crystal panel 12. In the secondembodiment, the light guide plate 22 is formed of acrylic and thethickness of the light guide plate 22 is 2 mm. Note that the light guideplate 22 may be formed of other materials such as polycarbonate. Thoughthe thickness of the light guide plate 22 is not limited to this value,the thickness thereof is set to a value ranging from 1 mm to 5 mm inorder to make the entire image display apparatus 10 thinner. The lightguide plate 22 is arranged in the rearward side of the liquid crystalpanel 12 in such a manner as to cover the entire rear surface side ofthe liquid crystal panel 12. Further, a dot-pattern 23 is formed on asurface opposite to the liquid crystal panel 12 of the light guide plate22, and the dot-pattern 23 functions to radiate the light incident froman end face 22 a of the light guide plate 22, to the liquid crystalpanel 12. The optical sheet 18 is formed such that a diffusion sheet, aprism sheet and a polarization separation sheet are stacked on eachother. The optical sheet 18 is arranged between the liquid crystal panel12 and the light guide plate 22.

The rear plate 24 is formed of a sheet metal having an outer shape ofrectangle larger in size than the light guide plate 22. The rear plate24 is arranged farther on a rearward side than the light guide plate 22.The reflection sheet 40, which is formed in a rectangular shape largerin size than the light guide plate 22, is arranged between the lightguide plate 22 and the rear plate 24.

The base plate unit 30 has a flexible substrate 32 and LED units 34. Theflexible substrate 32 is a base plate, having a general flexibility,which is formed such that polyimide film is used as an insulator andcopper foil is used as a conductor. Also, the flexible substrate 32 hasan overall length similar to that of the light guide plate 22 in thehorizontal direction of device. The flexible substrate 32 is formed suchthat the shape thereof is bent into an L-shape. A plurality of LED units34 are bonded to an outer surface of the flexible substrate 32. Each LEDunit 34 is provided with an LED 38.

A white light-emitting diode that radiates white light is used as theLED 38. More specifically, the LED 38 is structured such that a phosphorlayer, which radiates yellow light when excited by blue light, isstacked on a light-emitting face of a semiconductor device emitting bluelight. Thereby, white light which is the synthesized light of blue lightand yellow light radiates from the LED 38. It is appreciated here thatthe LED 38 may be structured such that a first phosphor layer, whichradiates red light when excited by blue light, and a second phosphorlayer, which radiates green light when excited by blue light, arestacked on the light-emitting face of a semiconductor device emittingblue light. By employing this modification, white light which is thesynthesized light of blue light, red light and green light can beobtained as well. To protect the LED 38 from the exterior environment,the LED unit 34 is sealed with a sealing material of optically low loadsuch as synthetic resin which is highly transparent in a visible range.

The flexible substrate 32 is arranged above the light guide plate 22such that the LED 38 is spaced apart from the end face 22 a of the lightguide plate by a distance H. Here, the distance H is a distance betweenthe LED 38 and the end face 22 a in a direction vertical to the end face22 a of the light guide plate 22. A light mixing chamber 42 is formed ina space surrounded by the end face 22 a of the light guide plate 22, thechassis 14 and the reflection sheet 40. The light mixing chamber 42functions to have light emitted from the LED 38 enter the end face 22 aof the light guide plate 22 while mitigating the luminancenon-uniformity. The light emitted from the LED 38 passes through thelight mixing chamber 42 and then directly enters the end face 22 a ofthe light guide plate 22 or enters the end face 22 a of the light guideplate 22 after having been reflected by an inner surface of the chassis14 or the reflection sheet 40. The light that has entered inside thelight guide plate 22 from the end face 22 a is then diffusely reflectedand irradiated to the rear surface of the liquid crystal panel 12through the optical sheet 18 as uniform light. At this time, the rearsurface of the light guide plate 22 is subjected to a processingtreatment so that light emitted from the LED 38 can be irradiated towardthe liquid crystal panel 12 at as close a vertical angle as possible.

The heatsink 26 is formed in a block-like shape and formed of a highlyheat-radiant material such as aluminum. Also, the heatsink 26 isarranged such that the heatsink 26 adheres tightly to the inner surfaceof the flexible substrate 32 to allow the heat generated by the LED 38to be released. The adhesion is maintained by the heat radiation sheet,silicon paste and the like, The heatsink 26 is fixed in front of therear plate 24 via the heat radiation sheet 28. The heatsink 26 and theheat radiation sheet 28 are thermally connected to each other with thesilicon paste applied therebetween. Though the flexible substrate 32 isused in the first embodiment, this should not be considered as limitingand, for example, a metal-based substrate may be employed.

The light shielding section 12 a of the liquid crystal panel 12 is soprovided as to cover an end face of the light guide plate 22 in frontthereof. The light shielding section 12 a is so provided that the endface 22 a is not seen from a user's virtual viewpoint S1 because the endface 22 a is blocked by the light shielding section 12 a. The virtualviewpoint S1 is a position assumed as an eye position when a user is toview an image displayed on the image display apparatus 10, and thevirtual viewpoint S1 is so determined as to lie within a predeterminedrange.

The virtual viewpoint S1 of the image display apparatus 10 as describedabove needs to cover exhaustively all ranges of the display of the imagedisplay apparatus 10 viewed by the user. The position assume as a user'seye position needs to not only include the front position of the imagedisplay apparatus 10 but also consider the case where a predeterminedangle is formed from the front position of the image display apparatus10. For example, when the image display apparatus 10 is viewed in thevertical direction, there are cases where a panel hanged from a ceilingor hanged on a wall is viewed from down below. Similarly, when the imagedisplay apparatus 10 is viewed in the horizontal direction, there arecases where it is viewed in an angled horizontal direction.

Further, the boundary in the rearward side of the light shieldingsection 12 a is defined to be a light shielding boundary S3. A rim in arange visible from the virtual viewpoint S1 in the rear surface of thelight guide plate 22, namely a spot that overlaps with the lightshielding boundary S3 as viewed from the virtual viewpoint S1 is definedto be a visible boundary S2.

FIG. 2 is a diagram showing the backlight device 20 according to thefirst embodiment as viewed from the viewpoint P of FIG. 1. Each LED unit34 has a package 50 and two LEDs 38. The package 50 is formed of ahighly heat-radiant semiconductor, and the outer shape of the package 50is formed in a rectangular plate-like shape. The package 50 is formedsuch that both ends of the package 50 in a longitudinal direction areraised and a center part thereof is recessed (recessed part). A bottom50 a of a recessed part is flat. The two LEDs 38 are placed side by sidein the longitudinal direction along the bottom 50 a at an interval of afirst placement interval P1.

An inclined portion 50 b is provided in each of the both ends in thelongitudinal direction in order that light radiating from side surfacesof the LED 38 can also be utilized efficiently. The height of theinclined portion 50 b is greater than that of the LED 38. Further, thesurface of the package 50 is formed such that the surface thereof has ahigher reflectance. Provision of the inclined portions 50 b as describedabove allows the light emitted from the LED 38 to enter at an anglecloser to the direction vertical to the end face 22 a. Conversely, theinclined portions 50 b blocks the light which radiates from the LED 38and travels at a large angle from the direction vertical to the lightguide plate 22.

In this manner, two LEDs 38 provided in each LED unit 34 are integrallystructured with each other. Note that the number of LEDs 38 provided ineach LED unit 34 is not limited to two and, for example, three or moreLEDs 38, such as three, four, six, or eight of them, may be provided. Insuch a case, too, a plurality of LEDs 38 provided in the same LED unit34 are placed side by side at equal intervals of the first placementinterval P1.

In the base plate unit 30, a plurality of LED units 34 are placed sideby side at equal placement intervals of P2 in a direction extendingalong the end face 22 a of the light guide plate 22. Here, anotherembodiment is conceivable where in order to increase the amount of lightincident on the light guide plate 22, the plurality of LED units 34 areplaced side by side such that the direction extending along the end face22 a forms a right angle relative to a direction in which the LEDs 38 ineach LED unit 34 are placed side by side. However, in the firstembodiment, a thin light guide 22 is used for the purpose of making theimage display apparatus 10 thinner. Accordingly, if the LEDs 38 arearranged according to the above conceivable embodiment, it will bedifficult to have the light emitted from the LEDs 38 enter the end face22 a properly.

Accordingly, in the first embodiment, a plurality of LED units 34 arearranged such that the LEDs 38 are aligned linearly in a directionextending along the end face 22 a. In the first embodiment, the firstplacement interval P1 is 0.6 mm, whereas the second placement intervalP2 is 10 mm.

It goes without saying that the first placement interval P1 and thesecond placement interval P2 are not limited to the above-mentionedvalues. The first placement interval P1 must be 0.1 mm or above in orderthat the LEDs 38 do not overlap in a pitch direction. Also, for eachceramic package, the packaging cost can be reduced if it can be mountedon a substrate using an automatic machine. In this manner, when theceramic package is to be mounted using the automatic machine, the firstplacement interval P1 needs to be 3 mm or below and is preferably 2 mm.Thus, the first placement interval P1 may be a value between 0.1 mm(inclusive) and 3 mm (inclusive). Also, the second placement interval P2may be a value between 5 mm (inclusive) and 15 mm (inclusive).

Each LED unit 34 is provided with an anode electrode and a cathodeelectrode. A pair of these electrodes is used as a common electrode, anda voltage is applied across these electrodes and thereby both the twoLEDs 38 in each LED unit 34 light up. The two LEDs 38 are connected inparallel with each other, and protection elements for protecting theLEDs 38 from excess voltage are also connected in parallel with eachother.

Where a thin light guide plate 22 is employed, it is difficult to placethe LEDs 38 side by side vertical to a direction extending along thelight guide plate 22, as described previously. For this reason, acountermeasure may be conceivable where a plurality of LEDs 38 arearranged by making the placement intervals of the LEDs 38 smaller inorder to avoid the insufficient amount of light incident on the lightguide plate 22. However, where the electrodes are provided for each LED38, the assembly work necessary for the mounting of the LED 38 may becumbersome and complicated, so that it may be difficult to suppress thenumber of steps required for the manufacturing of the image displayapparatus 10. In this manner, a plurality of LEDs 38 are provided ineach LED unit 34 and then the LED unit 34 is mounted on the flexiblesubstrate 32. Thus, a plurality of LEDs 38 can be mounted integrally.Accordingly, the installation job of the LEDs 38 can be done with easeand therefore the manufacturing process of the image display apparatus10 can be simplified.

It goes without saying that the preferred embodiments of the presentembodiment are not limited to a case where a plurality of LEDs 38 areintegrally configured as described above and, for example, a pair ofelectrodes to which a voltage is applied when the LEDs 38 light up maybe provided in each of the LEDs 38. Also, an arrangement may be asfollows. That is, a plurality of LEDs 38 are not configured integrally,and the placement interval of LEDs 38 in a group constituted by two ormore LEDs 38 may be the first placement interval P1, whereas theplacement interval between each of such groups may be the secondplacement interval P2.

The “group” defined herein is a group constituted by a plurality of LEDsarranged at equal intervals of the first placement interval P1. Thenumber of LEDs contained in each group is not limited to two and, forexample, it may be three or more LEDs, such as three, four, six, oreight of LEDs. In this case, a plurality of LEDs 38 may be arranged suchthat N×[first placement interval P1]<[second placement interval P2],where N indicates the number of LEDs included in each group.

The LED 38 functions as a point light source. The distribution D oflight emitted from each LED 38 is known to be circular as shown in FIG.2. Thus, there is a possibility that the light incident on the end face22 a of the light guide plate 22 may suffer the non-uniformity ofluminous intensity attributable to the placement intervals of the LED38. In the first embodiment, a difference between the maximum value andthe minimum value of the luminous intensity in the end face 22 a of thelight guide plate 22 is defined to be the “luminance non-uniformity”. Inthe luminous intensity distribution of light radiated from the LED unit34 according to the first embodiment, the luminous intensitydistribution component in a direction parallel to the end face 22 a ofthe light guide plate 22 is stronger than the luminous intensitydistribution component in a direction vertical to the end face 22 athereof. Such luminous intensity distributions as those may be a factorthat increases the non-uniformity of luminous intensity.

Since the luminous intensity distribution D of the LED 38 is asdescribed above, the luminance non-uniformity increases or decreases asthe distance H is varied. In general, as the spacing between the LEDsand the light guide plate becomes shorter, the decrease of luminousintensity between the LEDs becomes severer and the non-uniformity ofluminous intensity becomes larger. On the other hand, as the spacingbetween the LEDs and the light guide plate becomes longer and larger,the luminance non-uniformity becomes smaller but the average luminousintensity of light incident on the end face of the light guide platedrops due to the diffusion and absorption of light in the light mixingchamber 42. Thus, the distance H needs to be set so that the luminancenon-uniformity in the end face 22 a of the light guide plate 22 and theaverage luminous intensity of light incident on the end face 22 a of thelight guide plate 22 meet the respective required values.

If the LEDs 38 are all placed side by side at equal intervals, thedistance H that becomes minimum while the distance H is varied can beused. However, in the first embodiment, a plurality of LEDs 38 are suchthat the placement interval between two LEDs provided in each LED unit34 is the first placement interval P1, and the placement intervalbetween adjacent LED units 34 is the second placement interval P2.Accordingly, decrease or increase in the luminance non-uniformity isfurther caused depending on the first placement interval P1 and thesecond placement interval P2. As a result of research and development,it has been revealed that the degradation of the incident lightefficiency of light incident on the end face 22 a of the light guideplate 22 can be suppressed and, at the same time, the luminancenon-uniformity of light incident on the end face 22 thereof can beappropriately suppressed when the light guide plate 22 is so arranged asto meet a condition that P1≦H≦P2.

FIG. 3 is a graph showing a relation between the distance H and thenon-uniformity of luminous intensity K on the end face 22 a of the lightguide plate 22 in the backlight device 20 according to the firstembodiment. FIG. 3 shows the relations between the distance H and thenon-uniformity of luminous intensity K when the second placementinterval P2 is 5 mm, 10 mm, and 15 mm, respectively. In such cases, thedotted lines for the second placement interval P2 of 5 mm and 15 mmindicate non-uniformity widths when the first placement interval P1 isvaried in a range of 0.1 mm to 3 mm. FIG. 3 indicates that thenon-uniformity of luminous intensity K increases as the second placementinterval P2 increases.

A permissible luminance non-uniformity level Kmax is a limit value ofthe luminance non-uniformity of light incident on the end face 22 a ofthe light guide plate 22 to meet the required quality of the imagedisplay apparatus 10. Since the second placement interval P2 is 10 mm inthe first embodiment, the distance H is set to 4 mm which is a minimumvalue with which the non-uniformity of luminous intensity K becomes lessthan or equal to the permissible luminance non-uniformity level Kmax. Ascan be seen from FIG. 3, when a plurality of LEDs 38 are so arranged asto meet the condition that the first placement interval P1 ranges from0.1 mm to 3 mm and the second placement interval P2 ranges from 5 mm to15 mm, the distance H needs to be in a range of 1 mm to 7 mm to suppressthe non-uniformity of luminous intensity K at or below the permissibleluminance non-uniformity level Kmax. Thus, the light guide plate 22 isso arranged that the distance H ranges from 1 mm to 7 mm.

According to a preferred exemplary embodiment, the first placementinterval P1 is 0.5 mm, the second placement interval P2 is 5 mm, and thedistance H is 1.5 mm. According to another preferred exemplaryembodiment, the first placement interval P1 is 0.6 mm, the secondplacement interval P2 is 8 mm, and the distance H is 3 mm. According tostill another preferred exemplary embodiment, the first placementinterval P1 is 0.7 mm, the second placement interval P2 is 10 mm, andthe distance H is 4 mm.

Note that two LEDs 38 are mounted on each LED unit 34 and are soarranged as to meet a condition that 2×[first placement intervalP1]<[second placement interval P2]. In this case, the first placementinterval P1, the second placement interval P2 and the distance H are soset as to meet a condition that 1≦P2/2. In such a case, it is desirablethat the first placement interval P1 ranges from 0.1 to 2 mm, thedistance H from 1 mm to 5 mm, and the second placement interval P2 from5 mm to 11 mm.

FIG. 4 is an illustration schematically showing a front view of thebacklight device 20 according to the first embodiment. The light guideplate 22 is supported in lower two corners thereof by first supportingmember 60 and supported in upper two corners by second supportingmembers 62. As shown in FIG. 4, the base plate units 30 are providedsuch that the LEDs 38 are disposed counter to the end face 22 a at theupper side of the light guide plate 22 and are disposed counter to theend face 22 a at the lower side thereof, respectively. Also, the lightguide plate 22 is positioned by butting it against the first supportingmembers 60 by gravity. Thus, if height L4 of the light guide plate 22(hereinafter referred to as “light guide plate height L4” also) expandsor contracts due to changes in temperature of installation environment,the distance H between the lower-side end face 22 a and the LEDs 38 willvary little but the distance H between the upper-side end face 22 a andthe LED 38 will vary relatively greatly.

Now, refer back to FIG. 1. As the light guide plate 22 contracts due toa drop in the ambient temperature as described above, the distance Hbecomes large. Nevertheless, as the distance H becomes excessivelylarge, the end face 22 a of the light guide plate 22 reaches the visibleboundary S2 and thereby the end face 22 a of the light guide plate 22may be visible from the virtual viewpoint S1. When the user views theliquid crystal panel 12 from the front of the image display apparatus10, the end face 22 a of the light guide plate 22 is viewed as beingbrightly illuminated through the periphery of the light shieldingboundary S3. As a result, the periphery thereof appears dark, which maygive a sense of discomfort to the user.

For this reason, the light guide plate 22 according to the firstembodiment is so installed that the end face 22 a of the light guideplate 22 is shielded by the light shielding section 12 a and istherefore not seen from the user's virtual viewpoint S1, even when thelight guide plate 22 contracts due to changes in temperature within theassumed ambient temperature range. In actual setting, the lineconnecting the virtual viewpoint S1 and the visual boundary S2 does notbecome a straight line but it is assumed herein that the virtualviewpoint S1 and the visual boundary S2 can be connected by a straightline. Suppose, for example, that an angle relative to the horizontalline of a line-of-sight that views the light shielding boundary S3 fromthe virtual viewpoint S1 is denoted β and a distance from the lightshielding boundary S3 to the rear surface of the light guide plate 22 isdenoted L3. Then, the visual boundary S2 is located in a position higherthan the light shielding boundary S3 by L2 which is equal to L3×tan β.

For the image display apparatus 10, a standard ambient temperature Tswhich is considered normal in the assumed environment where the deviceis installed is set to 25 degrees. The light guide plate 22 is soprovided that the distance between the end face 22 a of the light guideplate 22 and the visual boundary S2 under the standard ambienttemperature Ts is a margin L1. Also, for the image display apparatus 10,an assumed ambient temperature which indicates a range of temperaturesassumed in the installation environment is set in advance. This assumedambient temperature is a temperature between a minimum ambienttemperature Tmin and a maximum ambient temperature Tmax. In the firstembodiment, the margin L1 is set such that L1 is longer than the lengthcorresponding to an amount of contraction observed when the ambienttemperature drops from the standard ambient temperature Ts to theminimum ambient temperature Tmin. Setting the margin L1 as describedabove prevents the end face 22 a from reaching the visible boundary S2in the event that the temperature reaches the minimum ambienttemperature Tmin.

Also, as the distance H becomes smaller as described above, theluminance non-uniformity of light incident on the end face 22 a mayincrease. Also, the light guide plate 22 is so provided that thedistance H is not shorter than a minimum distance Hmin at which theluminance non-uniformity attributable to the placement intervals in aplurality of LEDs 38 is a permissible value, even when the light guideplate 22 expands due to changes in temperature within the assumedambient temperature range. More specifically, the light guide plate 22is so provided as to meet a condition that [first placement intervalP1]≦[distance H]≦[second placement interval P2] even when the lightguide plate 22 expands or contracts due to changes in temperature withinthe assumed ambient temperature range.

FIG. 5 is a graph showing a relation between the light guide plateheight L4 and the distance H in the backlight device 20 according to thefirst embodiment. In FIG. 5, increasing straight lines indicate arelation between the light guide height L4 and the interval H at thetime the ambient temperature becomes the minimum ambient temperatureTmin. In FIG. 5, decreasing straight lines indicate a relation betweenthe light guide height L4 and the interval H at the time the ambienttemperature becomes the maximum ambient temperature Tmax. In FIG. 5, thedistance at the standard ambient temperature Ts is defined and denotedas Hs. A linear expansion coefficient α of material that may, forinstance, constitute the light guide plate 22 is as follows.

Acrylic: α=7 to 8×10⁻⁵ [/° C.]

Polycarbonate: α=6 to 7×10⁻⁵ [/° C.]

Thus, the linear expansion coefficient α of the light guide plate 22 isin a range of 6×10⁻⁵ to 8×10⁻⁵. FIG. 5 shows a relation between thelight guide height L4 and the distance H, about three kinds of lightguide plates 22 whose linear expansion coefficient α are 6×10⁻⁵, 7×10⁻⁵,and 8×10⁻⁵.

In FIG. 5, a maximum distance Hmax is the maximum value of distances Hat which the incident light efficiency of light incident on the end face22 a of the light guide plate 22 from the LEDs 38 is a permitted value.In this manner, the light guide plate 22 is so provided that thedistance H is not greater than the maximum distance Hmax at which theincident light efficiency of light incident on the end face 22 a of thelight guide plate 22 from a plurality of LEDs 38 is a permitted value,even when the light guide plate 22 contracts due to changes intemperature within the assumed ambient temperature range.

A minimum distance Hmin is the minimum value of distances H at which theluminance non-uniformity of light incident on the end face 22 a of thelight guide plate 22 from the LEDs 38 is a permitted value. As describedabove, the minimum distance Hmin is the minimum value that meets acondition that [first placement interval P1]≦[distance H]. Since thevalue of the minimum distance Hmin becomes the first placement intervalP1, the distance H is identical to the first placement interval in thecase of the first embodiment.

FIG. 5 indicates that in order not to exceed the maximum distance Hmaxin all ranges of the aforementioned linear expansion coefficients α inthe event that the distance H increases due to a contraction of thelight guide plate 22, the light guide plate height L4 needs to be 800 mmor below. Also, in order not to fall below the minimum distance Hmin inall ranges of the aforementioned linear expansion coefficients α in theevent that the distance H decreases due to an expansion of the lightguide plate 22, the light guide plate height L4 needs similarly to be800 mm or below.

Even when the light guide plate 22 contracts maximally due to changes intemperature within the assumed ambient temperature range, the maximuminterval Hmax in FIG. 5 may be the maximum value of the distances H atwhich the end face 22 a of the light guide plate 22 is shielded by thelight shielding section 12 a of the liquid crystal panel 12 and istherefore not seen from the user's virtual viewpoint S1. In this case,the sum of the distance H and the margin L1 at the standard ambienttemperature Ts needs to be the maximum distance Hmax or below.

FIG. 6 is a block diagram schematically showing a configuration of alighting control system 70 provided in the backlight device 20 accordingto the first embodiment. The lighting control system 70 includes an LEDdrive control circuit 72, and LED drive circuit 74, an LED 38, and atemperature sensor 76. The temperature sensor 76 is provided inside theimage display apparatus 10, and the temperature sensor 76 detects theambient temperature around the light guide plate 22. The temperaturesensor 76 is connected to the LED drive control circuit 72, which readsa detection result of the temperature sensor 76 for every predeterminedtime interval. The LED drive control circuit 72 sends a control signalto the LED drive circuit 74, which supplies a drive currentcorresponding to the received control signal, to the LED 38.

For example, suppose that the standard ambient temperature Ts is 25° C.,the maximum ambient temperature Tmax is 500° C., the light guide plate22 is formed of acrylic whose linear expansion coefficient α is 8×10⁻⁵,and the light guide plate height L4 is 400 mm. Then, the distance Hcontracts by 0.8 mm which is equal to 400×25×(8×10⁻⁵) after the ambienttemperature has reached the maximum ambient temperature Tmax. If thedistance H is 4.8 mm at the standard ambient temperature Ts, thedistance H will be 4.0 mm at the maximum ambient temperature Tmax. Asthe distance H becomes smaller as described above, the incident lightefficiency of light incident on the light guide plate 22 increases. Thelighting control system 70 controls the power to be supplied to the LED38 to suppress the non-uniformity in the luminance of light, irradiatedto the liquid crystal panel 12 from the backlight device 20, caused bysuch changes of the light incident efficiency.

More specifically, the LED drive control circuit 72 controls the powersupplied to the LED 38 in the following manner. That is, as the ambienttemperature around the light guide plate detected by the temperaturesensor 76 increases, the drive current supplied to the LED 38 isdecreased; as the ambient temperature around the light guide plate 22drops, the drive current supplied to the LED 38 is increased. Forexample, when the temperature rises by x % relative to the standardambient temperature Ts, the drive current supplied to the LED 38 isreduced by axx % where “a” is a constant. Conversely, when thetemperature drops by x % relative to the standard ambient temperatureTs, the drive current supplied to the LED 38 is increased by axx %. As aresult, the LED 38 can be lit efficiently. Thus the power consumed bythe LED 38 can be reduced and the amount of heat generated by the LED 38can be suppressed.

Note that the output value of the temperature sensor such as athermistor varies logarithmically in relation to changes in temperature.Accordingly, the LED drive control circuit 72 may store a table thatdefines a correspondence relation between the output values of thetemperature sensor 76 and the drive current to be supplied to the LED38. The LED drive control circuit 72, when it has read the output valueof the temperature sensor 76, may determine the drive current to besupplied to the LED by referencing the table.

The light guide plate 102 has rectangular grooves 102 b approximately inthe centers of left-hand-side and right-hand-side end faces of the lightguide plate 102. The rear plate 24 is fixed to two supporting members104. The outer shape of each supporting member 104 is a rectangle andhas a height slightly smaller than the depth of the groove 102 b. Thelight guide plate 102 is so arranged that the supporting member 104 isinserted in the groove 102 b. In this manner, the light guide plate 102is positioned by the supporting members 104 approximately in the centeralong the direction of the light guide plate height L4. Note that theouter shape of the grooves 102 and the supporting members 104 is notlimited to the rectangle.

As described above, the light guide plate 102 is positionedapproximately in the centers along the height direction. Thus, thenon-uniformity in one of the distances H in the vertical direction canbe suppressed by about half as compared with the case where one of theend faces in the vertical direction is positioned as in the firstembodiment, for instance. Note that the height at which the light guideplate 102 is positioned is not limited to an approximately centralposition along the height direction and, for example, the light guideplate 102 may be positioned in any other midway position or any otherarbitrary position of left-hand-side and right-hand-side end faces ofthe light guide plate 102.

The present invention is not limited to the above-described embodimentsonly, and any combination of the above-described structural componentsas appropriate is effective as and encompassed by the presentembodiments. Also, it is understood by those skilled in the art thatvarious modifications such as changes in design may be made based ontheir knowledge and the embodiments added with such modifications arealso within the scope of the present invention.

EXPLANATION OF REFERENCE NUMERALS

10 Image display apparatus

12 Liquid crystal panel

12 a Light shielding section

13 Front cover

14 Chassis

20 Backlight device

22 Light guide plate

22 a End face

23 Dot-pattern

24 Rear plate

26 Heatsink

30 Base plate unit

32 Flexible substrate

34 LED unit

38 LED

40 Reflection sheet

42 Light mixing chamber

50 Package

50 a Bottom

50 b Inclined portion

60 First support member

62 Second support member

70 Lighting control system

72 LED drive control circuit

74 LED drive circuit

76 Temperature sensor

100 Backlight device

102 Light guide plate

102 a End face

INDUSTRIAL APPLICABILITY

The present invention is usable for a backlight device and particularlyusable for a backlight device that irradiates light from a rear surfaceof an image display panel.

1. A backlight device, comprising: a light guide plate; and a pluralityof light source units, placed side by side at placement intervals of P2in a direction extending along an end face of the light guide plate, theplurality of light source units being spaced apart from the end face ofthe light guide plate by a distance H, wherein each of the plurality oflight source units includes: a plurality of white light-emitting diodesplaced side by side at placement intervals of P1 in the directionextending therealong; and a package section, on which the plurality ofwhite light-emitting diodes are fixed, having a pair of inclinedportions provided at both ends in the direction extending therealong,wherein the pair of inclined portions reflect white light radiated fromthe plurality of white light-emitting diodes so as to be guided to theend face thereof, and the height of the inclined portion is greater thanthat of the light-emitting diode, and wherein the light guide plate isso arranged as to meet a condition that P1≦H≦P2.
 2. A backlight deviceaccording to claim 1, wherein the plurality of light source units areplaced side by side in such a manner as to meet condition that P2 is ina range of 5 mm to 15 mm, wherein the plurality of white light-emittingdiodes are placed side by side in such a manner as to meet a conditionthat P1 is in a range of 0.1 mm to 3 mm, and wherein the light guideplate is placed in such a manner as to meet a condition that H is in arange of 1 mm to 7 mm.
 3. A backlight device according to claim 1,wherein the thickness of the light guide plate is in a range of 1 mm to5 mm.
 4. (canceled)
 5. A backlight device according to claim 1, whereinthe plurality of white light-emitting diodes are placed such thatN×P1<P2, where N indicates the number of white light-emitting diodesincluded in each of the light source units.